1. Field of the Invention
This invention relates generally to cast metal turbine wheels with integral airfoil blades, and more particularly to accommodation of contraction during solidification of the cast material.
2. Description of the Prior Art
Various methods have been devised for producing gas turbine wheel by casting metal. Some of these methods use a directional solidification approach for the blades in order to provide a columnar grain structure oriented generally radially with respect to the wheel axis and, accordingly, substantially parallel to the leading and trailing edges of each blade. Examples are shown and described in U.S. Pat. No. 4,240,495 issued Dec. 23, 1980, and U.S. Pat. No. 4,436,485 issued Mar. 13, 1984 to George L. Vonnegut. Both of these patents disclose chill material in the mold around the perimeter of the blade tip circle so that, as the cast material cools, the heat transfer is radially outward from the blade tip portions of the mold into the chill material. The result is establishment of the grains of the solidifying metal from the blade tips inwardly in a columnar fashion. In short, the blades are solidified unidirectionally from outside to inside by withdrawing heat through the chill material located immediately radially outboard of the blade tip portion of the mold cavity. Because of the arrangement of the mold, there is a relatively large mass of material in the disk forming portion of the mold cavity. Grain growth in the disk portion was isotropic, and equiaxed grains were formed in that portion of the wheel.
As is often true in metal casting, shrinkage during cooling can create some problems. At least one of those problems is development of high stresses and tears or cracks. Another is a tendency of the outer portion of the casting at the blade tips to move away from the chill material in the mold and thus detract from the heat transfer to the chill material. But efforts to incorporate a chill inboard of the blades in order to accomplish directional solidification outward from a central aperture usually have been frustrated prior to the present invention, because contraction onto a central chill has been even more conducive to high stresses and hot tears or cracks. Such stresses may also contribute to recrystallization upon subsequent high temperature heat treatment of the turbine wheel. The same effect ay also occur as a result of high temperatures in actual use of the wheel in a gas turbine assembly.